1. Field of the Invention
The present invention relates to a cellular communication system, and more particularly, to a method and system for providing a power-efficient soft handoff in a mobile station which can increase power efficiency by using characteristics of a multicarrier code division multiple access(hereinafter, referred to as CDMA) cellular system.
2. Description of the Prior Art
In a prior art which is directed to a soft handoff in a mobile station in a CDMA cellular system, disclosed is a U.S. Pat. No. 5,101,501, entitled "Method and System for Providing a Soft Handoff in Communications in a CDMA Cellular Telephone System". An object of this patent is to provide a soft handoff method and system in a system using a single carrier. That is, since all base stations in a CDMA cellular system transmit the same frequency signal, a mobile station can receive all signals transmitted from the base stations.
Moreover, since each base station uses the same code having a long period, but spectrum-spreads information with the code which has different phase from each other, the mobile station can measure a received strength of a pilot signal in each of the base stations where encryption is not achieved and only a symbol "0" is spectrum-spread with code search at a time. Hence, during the mobile station communicates with other user through one base station, it can measure the strength of pilot signal in the adjacent base station to the base station to thereby determine its own handoff time point. Of course, the mobile station should separately include a receiver for performing a continual communication with one base station and a searcher for measuring a received strength of the pilot signal in each base station.
By the reasons as discussed above, if the mobile station can know user encryption information, it can demodulate an information channel signal in any base station. Therefore, when two base stations transmit the same information to one mobile station, the one mobile station can demodulate the received information from the two base stations.
At the time, the mobile station should of course include a separate receiver for each receiving the received signals from the two base stations. Using the characteristics of CDMA system, until a handoff operation from one base station to the other base station is started and completed, if the two base stations transmit the same information to the mobile station which is in a handoff state, the mobile station receives the signals transmitted from the two base stations and combines the received signals to demodulate the combined signals.
The mobile station measures a received strength of a pilot signal from an adjacent base station to the one base station. If the received strength of the pilot signal of the one base station is higher than that of the adjacent base station, the mobile station determines that its own handoff is completed. Then, the mobile station modulates only the information transmitted from the base station where the received strength of pilot sinal is highest and then requests communication stop.
Therefore, the mobile station can execute a safe handoff, without cutting communication with the base station. This is called "soft handoff" or "make-before-break".
The mobile station includes two receivers which receive the signals transmitted from two base stations, and upon performing the soft handoff, further includes a separate receiver which a multipath combining receiver(RAKE receiver) which combines multipath signal components of the received signal from one base station by using a direct sequence spread spectrum modulation.
As a result, from the time when the mobile station starts the soft handoff to the time it completes the soft handoff, the mobile station performs the diversity combining operation of the multipath signal components of each base station as well as performs the diversity combining operation of signals received from the two base stations, which enhances receiving performance.
In this case, however, the mobile station does not a separate receiver which is used only upon the soft handoff, but, among receivers which are used as the multipath combining receiver upon a normal operation, uses a part of the receivers to receive the signals transmitted from another base stations during the soft handoff.
Therefore, since the mobile station can not combine the multipath signal components from two base stations during the soft handoff, to compensate this defect, the two base stations should transmit higher power to the mobile station, when compared with the mobile station having the separate receiver that combines the multipath signal components from the two base stations.
As a result, all of the mobile stations receive a large number of interference signals, when compared with the case where the soft handoff is not executed. Hence, there occurs a disadvantage in that the base station decreases capacity of a forward link as a communication link of the mobile station.